Photonic integrated circuit

ABSTRACT

Methods, systems, and apparatus, including an optical receiver including an optical source, including a substrate; a laser provided on the substrate, the laser having first and second sides and outputting first light from the first side and second light from the second side, the first light output from the first side of the laser has a first power and the second light output from the second side has a second power; and a first modulator that receives the first light and a second modulator that receives the second light, such that the power of the first light at an input of the first modulator is substantially equal to the power of the second light at an input of the second modulator.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application claims the benefit of U.S. Provisional PatentApplication No. 62/274,377, filed Jan. 4, 2016, and U.S. ProvisionalPatent Application No. 62/379,682, filed Aug. 25, 2016 which areincorporated by reference herein.

TECHNICAL FIELD

The present disclosure is directed to a control of a tunable laserimplemented on a photonic integrated circuit. For example, a laser maybe integrated on an optical transmitter to transmit an optical signal.As another example, a laser may be integrated on a coherent opticalreceiver as a local oscillator source for detecting an incoming opticalsignal in a coherent manner.

SUMMARY

In a general aspect, the subject matter described in this specificationcan be embodied in an optical receiver including an optical source,including a substrate; a laser provided on the substrate, the laserhaving first and second sides and outputting first light from the firstside and second light from the second side, the first light output fromthe first side of the laser has a first power and the second lightoutput from the second side has a second power; and a first modulatorthat receives the first light and a second modulator that receives thesecond light, such that the power of the first light at an input of thefirst modulator is substantially equal to the power of the second lightat an input of the second modulator.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other potentialfeatures and advantages will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a block diagram of an optical communication consistent withthe present disclosure;

FIG. 2a illustrates an example of transmitter block consistent with anaspect of the present disclosure.

FIGS. 2b-2e show examples of optical sources having balanced outputpowers consistent with an aspect of the present disclosure;

FIGS. 3a-3d show examples of granting teeth and correspondingreflectivity characteristics consistent with an additional aspect of thepresent disclosure;

FIG. 4 shows a block diagram of an example receiver consistent with thepresent disclosure.

FIGS. 5a-5h show examples of optical receiver circuits consistent withadditional aspects of the present disclosure;

FIGS. 6-7 shows a block diagram of an example optical receiver circuithaving a shutter variable optical attenuator;

FIGS. 8-9 illustrate examples of a photonic integrated circuit having ashutter variable optical attenuator consistent with further aspects ofthe present disclosure; and

FIG. 10 shows an example of a communication system having an N×Nconfiguration consistent with an additional aspect of the presentdisclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the present exemplaryembodiments of the present disclosure, which are illustrated in theaccompanying drawings. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or like parts.

A tunable laser enables an operating wavelength of a laser to beadjusted over a tunable wavelength range. Tunable lasers such assemiconductor laser diodes typically have a gain section and an optionalphase section provided between a pair of reflectors or mirrors. The gainsection includes a p-n junction, and the phase section adjusts the phaseof light in the laser cavity between the reflectors. A reflector may bea grating-based reflector, which includes a waveguide having a periodicrefractive index variation (or grating) corresponding to a particularwavelength of light output from the laser. For example, the reflectorshave a reflectivity characteristic that includes a series of uniformlyspaced reflection peaks, which resemble a comb. The spectral distancebetween successive peaks in the comb or pitch of one reflector may bedifferent than the spectral distance between successive peaks of theother reflector. Each “comb” may be spectrally shifted by tuning thereflectors and phase sections to select a single wavelength over a widerange, such as the C-Band (1530-1565 nm), L-Band (1565-1625 nm),extended C-Band, or extended L-Band. The grating-based reflector may bea partial reflector or a total reflector.

In some implementations, the grating-based reflectors may be used totune the wavelength or the power of light output from the laser. Forexample, an operating wavelength of a laser may be tuned using heatersthat are provided above and/or adjacent to the grating-based reflectors.The heaters adjust the temperature of the grating-based reflectors, suchthat the effective pitch of the periodic refractive index variationchanges. The change in the optical pitch of the periodic refractiveindex variation changes the reflectivity characteristic of thereflector, and as a result may change the wavelength or the power oflight output from the laser.

Tunable semiconductor lasers may be provided on or integrated with otheroptical devices on a common substrate, as a photonic integrated circuit(PIC). Such other devices include, but are not limited to, waveguides,modulators (e.g., Mach-Zehnder (MZ) modulators), couplers (e.g.,multimode interference (MMI) couplers), optical combiners, splitters,multiplexers, demultiplexers, variable optical attenuators,semiconductor optical amplifiers, optical hybrids, and photodiodes. Thelaser may be configured such that light output from both sides or facetsof the laser is supplied to a respective one of these devices. Often,the polarization of light output from one side of the laser is rotatedrelative to the polarization of light output from the other side of thelaser following optical processing, such as modulation, of the laserlight. In one example, the light output from both sides of the laser hasa transverse electric (TE) polarization and the polarization rotatedoptical signal has a transverse magnetic (TM) polarization. Theresulting optical signals, each having a different polarization, maythen be combined to provide a polarization multiplexed optical signalhaving a higher capacity than optical signal having a singlepolarization.

Light output from one side of the laser, however, may propagate over adifferent distance or over a different optical path in the PIC or maytravel through different devices than light output from the other sideof the laser. Accordingly, the optical loss experienced by light outputfrom one side of the laser may be different than the loss experienced bylight output from the other side of the laser. As a result, followingpolarization rotation and multiplexing the TE polarized component of themultiplexed optical signal may have a power level that is substantiallydifferent than the TM polarized component of the optical signal. Suchpower imbalances in the polarization multiplexed optical signal maycause errors when the signal is detected. In addition, the higher powerpolarization component may be preferentially amplified duringpropagation through a chain of optical amplifiers, such that the lowpower polarization component of the optical signal receives less powerat each amplifier and thus suffers from a lower optical signal-to-noiseratio (OSNR).

In one example of this disclosure, an optical device, which may be apassive device, such as an optical coupler, is provided in one opticalpath carrying a higher power optical signal. Alternatively, thereflector reflectivities may be adjusted such that light output from oneside of the laser that experiences more loss is output from a reflectorhaving a lower reflectivity, and thus higher power, than light outputfrom the other side of the laser.

Moreover, it may be desirable to test a photonic integrated circuit(“PIC”) before packaging in order to improve a yield of a packagedoptical module or system. Accordingly, consistent with a further aspectof this disclosure, implementations for on-chip testing of the tunablelasers and other optical components on the PIC are provided. Forexample, a “shutter” variable optical attenuator (“SVOA”) may be used totransmit a tapped signal for testing in a testing mode, and to block thetapped signal in an operation mode. By incorporating the SVOA in thePIC, there is no need to allocate an additional output port forproviding an optical signal for testing. Thus, the PIC may include moredevices than would otherwise be provided or have a smaller footprint. Inanother case, a shutter VOA may be used to allow different amounts oflight to transmit through for testing mode and for normal operation. Forinstance, this may allow sufficient power for rapid wavelengthcharacterization of the laser in test mode, and adequate attenuation toensure that the laser linewidth is minimized in normal operation overC-band. In another example, an alignment laser may be further beincorporated in the PIC, such that a coupling between the PIC and anexternal waveguide or optical fiber may be improved. In a furtherexample, a loopback waveguide may be provided on the PIC in order totest fabricated devices before dicing. Accordingly, wafer-level testingmay be achieved.

Examples of power balanced lasers will next be described with referenceto FIGS. 1, 2 a-2 e, 3 a-3 d, 4, and 5 a-5 f, and various SVOAimplementations are described below with reference to FIGS. 6-11.

FIG. 1 illustrates an optical link or optical communication system 100consistent with an aspect of the present disclosure. Opticalcommunication system 100 includes a plurality of transmitter blocks (TxBlock) 12-1 to 12-n provided in a transmit node 11. Each of transmitterblocks 12-1 to 12-n receives a corresponding one of a plurality of dataor information streams Data-1 to Data-n, and, in response to arespective one of these data streams, each of transmitter blocks 12-1 to12-n may output a group of optical signals or channels to a combiner ormultiplexer 14. Each optical signal carries an information stream ordata corresponding to each of data streams Data-1 to Data-n. Multiplexer14, which may include one or more optical filters, for example, combineseach of group of optical signals onto optical communication path 16.Optical communication path 16 may include one or more segments ofoptical fiber and optical amplifiers, for example, to optically amplifyor boost the power of the transmitted optical signals.

As further shown in FIG. 1, a receive node 18 is provided that includesan optical splitter or demultiplexer 20, which may include one or moreoptical filters, for example, optical demultiplexer 20 supplies eachgroup of received optical signals to a corresponding one of receiverblocks (Rx Blocks) 22-1 to 22-n. Each of receiver blocks 22-1 to 22-n,in turn, supplies a corresponding copy of data or information streamsData-1 to Data-n in response to the optical signals. It is understoodthat each of transmitter blocks 12-1 to 12-n has the same or similarstructure and each of receiver blocks 22-1 to 22-n has the same orsimilar structure. In some implementations, there may be multiplemultiplexers and fibers and wavelength selective switches and ROADMsemployed in the link.

FIG. 2a illustrates one of transmitter blocks 12-1 in greater detail.Transmitter block 12-1 may include circuitry (not shown) that processeseach of data streams Data-1 to Data-n and supplies electrical signal(including drive signals) to optical sources or transmitters OS-1 toOS-2. These electrical signals are used to drive modulators in eachoptical source to provide modulated optical signals corresponding todata streams Data-1 to Data-n. Such modulators are discussed in greaterdetail below.

In one example, optical sources OS-1 to OS-n are provided on transmitphotonic integrated circuit (PIC) 205 provided on a substrate, includingfor example, a group III-V material, such as indium phosphide, orsilicon or other semiconductor or insulative material. As further shownin FIG. 2a , each of optical sources OS-1 to OS-n supplies acorresponding pair of modulated optical signals (for example, arespective one of pairs λ1TE, λ1TE′ . . . λnTE, λnTE′) to wavelengthmultiplexing circuitry 208. Typically, each optical signal within agiven pair has the same or substantially the same wavelength, e.g., eachof optical signals λ1TE, λ1TE′ have wavelength λ1. In one example, eachof optical signals λ1TE to λnTE are multiplexed by wavelengthmultiplexing circuitry 208 into a first WDM output 290 and each ofoptical signals λ1TE′ to λnTE′ are multiplexed into a second WDM output291. Wavelength multiplexing circuitry 208 may include one or moreoptical combiners or arrayed waveguide gratings (AWGs) and/or one ormore power combiners. Optical sources OS-1 to OS-10 and wavelengthmultiplexing circuitry 208 may be provided on substrate 205, forexample. Substrate 205 may include indium phosphide or othersemiconductor materials.

As further shown in FIG. 2a , the first (290) WDM output (includingoptical signals λ1TE to λnTE) may be provided to polarizationmultiplexing circuitry 295, including for example a polarization beamcombiner. In one example, each optical signal in first WDM output 290may have a transverse electric (TE) polarization and is supplied to apolarization beam combiner by polarization maintaining optical fiber290-1, such that the polarization of each optical signal in the firstWDM output has the TE polarization upon input to polarizationmultiplexing circuitry 295. Each optical signal in WDM output 290therefore has a TE designation. The second WDM output 291 may also havea TE polarization when output from wavelength multiplexer 208 (andtherefore each optical signal λ1TE′ to λnTE′ in WDM output 291 has aTE′), but the second WDM output 291 may be provided to a secondpolarization maintaining fiber 291-1 that is twisted in such a way thatthe polarization of each optical signal in the second WDM output 291 isrotated, for example, by 90 degrees. Alternatively, a rotator 294 may beprovided as shown in FIG. 2a to rotate the polarization of each ofoptical signals λ1TE′ to λnTE′. Accordingly, after such rotation, eachsuch optical signal may have a transverse magnetic (TM) polarizationwhen supplied to polarization multiplexing circuitry 295. Polarizationmultiplexing circuitry 295, in turn, combines the two WDM opticaloutputs to provide a polarization multiplexed WDM optical signal 296λ1(TE+TM) to λn(TE+TM).

It is understood, that optical sources OS-1 to OS-10, as well as thewavelength multiplexing circuitry, wavelength multiplexer or wavelengthcombiner 208, may be provided as discrete components, as opposed tobeing integrated onto substrate 205. Alternatively, selected componentsmay be provided on a first substrate while others may be provided on oneor more additional substrates in a hybrid scheme in which the componentsare neither integrated onto one substrate nor provided as discretedevices.

FIG. 2b illustrates an example of optical source OS-1 consistent with anaspect of the present disclosure. It is noted that each of remainingoptical sources OS-2 to OS-n shown in FIG. 2a may have the same orsimilar structure as optical source OS-1 shown in FIG. 2b . Opticalsource OS-1 has a laser including mirror or reflector sections 202 and208 that may define a cavity of laser 210. Gain (204) and phaseadjusting (209) sections may be provided between reflector sections 202and 208. Light, e.g., continuous wave (CW) light, 232 may be suppliedfrom side S1 of laser 210 to modulator 218 and CW light 234 may besupplied from side S2 of laser 210 to modulator 222 via variable opticalattenuator (VOA) 237. As noted above, the power output from one side,such as side S1, of the semiconductor laser, such as laser 210, maydiffer from the power of light output from the other side of laser, suchas side S2 before or after the modulator. Accordingly, consistent withan aspect of the present disclosure, a variable optical attenuator (VOA)237 may be provided that receives light output from side S2 of laser 210to selectively attenuate such light so to be at a desired level. Asfurther noted above, the power of light output from side S2 may beadjusted or power balanced by the VOA 237 to the same or substantiallythe same as the light output from side S1.

Alternatively, the power of light output from side S2 may be adjusted orpower balanced so that such light has the same power at modulator 222 asthe power of light output from side S1 at modulator 218 if loss afterthe modulator is the same. In another example, light propagating from S1may experience higher design loss than light propagating from S2 due toany combination of a longer waveguide path, a lossier waveguide path,similar or additional optical components with higher insertion lossbefore and after the modulators, or different optical gain before orafter the modulator, and so the loss compensation may balance the entirenet loss difference. In another example, the power of the twopolarizations for the same wavelength that is transmitted down the fiber(e.g., λ1 (TE+TM) as shown in FIG. 2a ) may experience different lossesfrom the laser to the fiber. The loss compensation may balance theentire net loss difference between the two polarizations along theseparate optical paths.

VOA 237, however, is typically biased and thus may consume power.Accordingly, consistent with an additional aspect of the presentdisclosure, VOA 237 may be replaced with a passive device, e.g., onethat does not have a current or voltage applied thereto, such as coupler231 (see FIG. 2c ). Coupler 231 may be configured or have a structuresuch that light output from side S2 of laser 210 may incur apredetermined loss to achieve the power balancing noted above.

In one example, coupler 231 may be an MMI coupler, which has a port orlocation 231-1 at which extraneous light may be output. In order toprevent or limit such light from being reflected back to the laser, forexample, a low loss or dispersive structure may be provided at suchlocation 231-1. Here, such structure constitute waveguide 249, which mayhave a spiral shape. Other waveguides shapes and low loss or dispersivestructures may be provided. Such structures are described in U.S. PatentApplication Publication No. 2014/0185979, the entire contents of whichare incorporated herein by reference. Alternatively, a photodiode may beused to absorb the light and the photocurrent may be used to monitor theoptical power. Referring to FIG. 2d as an example, light from the port231-1 is provided to a photodetector 236, where the light is absorbedand photocurrent is generated.

Power balancing consistent with a further aspect of the presentdisclosure will next be described with reference to FIG. 2d . Here, adevice, such as coupler 231, that imparts a predetermined loss in thepath of the output light is omitted, and the reflectivities ofreflectors 202 and 208 are designed to provide the desired power levelsof the light output from sides S1 and S2. For example, the reflectivityof reflector 202 may be adjusted to be relatively high, so that lesslight or light 232 with less power is output from side S1. On the otherhand, reflector 208 may be designed to have a relatively lowreflectivity, such that light 234 output from side S2 has more power. Asa result, if the loss encountered by light output from side S2 along apath to modulator 222, for example, is greater than the loss encounteredby light output from side S1 along a path to or after modulator 218, thepower adjustment noted above may insure that that such light supplied tomodulators 218 and 222 or after the modulators is the same orsubstantially the same.

Reflectively adjustment will next be described with reference to FIGS.3a -3 d. As noted above, the reflector or mirror sections of the laser,such as mirror section 202 and 208 may include a plurality of refractiveindex variations which collectively constitute a grating in each mirrorsection. Such refractive index variations in mirror section 202 areshown in FIG. 3a as a plurality of grating “teeth” or high refractiveportions 310 spaced from each other by low refractive index portions312. As shown in FIG. 3b , such refractive index variations yield areflectivity characteristic of reflector section 202 having a series ofpeaks, each at a respective wavelength (λ1, λ2, λ3), and each having amaximum reflectivity M_(H). Reflector section 208, on the other hand,has a fewer teeth or high refractive index portions 314 spaced from eachother by low refractive index portions 316 (see FIG. 3c ). The resultingreflectivity characteristic 313 shown in FIG. 3d also has reflectivitypeaks, but such peaks are at different wavelengths (λ1′, λ2′, λ3′).Moreover, the peaks in FIG. 3d have a lower maximum reflectivity (M_(L))than that of the peaks shown in FIG. 3b . That is, by forming reflectorsection 208 with fewer refractive index teeth than reflector section202, the reflectivity of section 208 at wavelengths λ1′, λ2′, λ3′ isless than the reflectivity of section 202 at wavelengths λ1′, λ2′, λ3′.Accordingly, the reflectivity of the reflector sections of laser 210 canbe in part controlled based on the number of refractive index teeth.

Returning to FIG. 1, a multiplexed optical signal is transmitted to areceive node 18, which includes a demultiplexer 20. Demultiplexer 20outputs groups of optical signals that make up the multiplexed signal,such that each group is provided to a corresponding one of receiverblocks receiver (Rx) blocks 22-1 to 22-n. One such receiver block, Rxblock 22-1 will next be described with reference to FIG. 4. It isunderstood that remaining receiver blocks 22-2 to 22-n have the same orsimilar construction as receiver block 22-1.

Referring to FIG. 4, Rx block 22-1 may include a polarization beamsplitter (PBS) 402 that receives polarization multiplexed opticalsignals λ1(TE+TM) to λn(TE+TM) from demultiplexer 20 and providesoptical signals λ1TE to λnTE having a TE polarization at a first output402-1 and optical signals λ1TM to λnTM having a TM polarization at asecond output 402-2. Optical signals λ1TM to λnTM may be fed to apolarization rotator 404 that rotates the polarization of each suchoptical signal from the TM polarization to the TE polarization. Suchpolarization rotated signals are therefore designated λ1TE′ to λnTE′ inthe drawings.

As further shown in FIG. 4, optical signals λ1TE to λnTE and λ1TE′ toλnTE′ are next provided to wavelength demultiplexer or splitter circuit410, which outputs pairs of optical signals, such that each opticalsignal in each such pair has the same wavelength. Accordingly, forexample, wavelength demutiplexing or splitter circuit 410 suppliesoptical signals λ1TE and λ1TE′ at a respective one of outputs 410-1,410-2. Each optical signal pair, in turn, is provided to a correspondingone of optical receiver circuits OR-1 to OR-n. Optical receiver circuitsOR-1 to OR-n and wavelength demultiplexer 490 may be integrated in PIC408 on substrate 406. Alternatively, wavelength demultiplexer 490 andoptical receiver circuits may be provided as discrete components or onseparate substrates.

One of optical receiver circuits, OR-1, will next be described withreference to FIGS. 5a-5h . It is understood that each of remainingoptical receiver circuits OR-2 to OR-n have the same or similarstructure as optical receiver circuit OR-1.

Consistent with an aspect of the present disclosure, optical signalsλ1TE and λ1TE′, as well as optical signals λ2TE to λnTE and λ2TE′ toλnTE′ may be modulated, for example, in accordance with a phasemodulation format, such as BPSK, QPSK, higher-order QAM or another phasemodulation format. Accordingly, coherent detection may be employed inorder to appropriately sense and extract the data carried by theseoptical signals. FIG. 5a shows an example of optical receiver circuitOR-1, which may be suitable to carry out coherent detection consistentwith an aspect of the present disclosure.

In particular, as further shown in FIG. 5a , OR-1 may include a localoscillator (“LO”) laser 502, first and second photodiode arrays 508-1and 508-2, and a control 514. The LO laser 502 is similar to or the sameas laser 210 described above with reference to FIGS. 2b-2d but isconfigured to generate light having a reference wavelength. For example,laser 502 may also have first and second reflector sections, a gainsection, and a phase adjusting section. In addition, as discussed ingreater detail below, the reflector sections of laser 502 may havedifferent reflectivity in order to achieve power balancing in a mannersimilar to or the same as that described above in regard to laser 210.

In general, light output from side S1 of LO laser 502 is mixed withincoming optical signal λ1TE in optical hybrid 506-1 to generate one ormore mixed AC output signals or mixing products, which are then suppliedby optical hybrid circuit 506-1 to photodetector or photodiode array508-1. Photodetector array 508-1 may detect an AC signal in the mixingproducts to generate an AC photocurrent that may be further processed torecover data carried by optical signal λ1TE. Similarly, light outputfrom side S2 of laser 502 may be mixed with optical signal λ1TE′ inoptical hybrid circuit 506-2 to provide mixing products, which are fedto photodetector or photodiode array 508-2. Photodetector array 508-2may detect an AC signal in the mixing products supplied thereto togenerate an AC photocurrent that may also be further processed torecover data carried by optical signal λ1TE′.

Preferably, the AC photocurrent generated by photodetector array 508-1is the same or substantially the same as the AC photocurrent generatedby photodetector array 508-2. Such AC photocurrent may be defined as:

AC Iph=√{square root over (Iph,signal·Iph,LO)},

where “AC Iph” is the generated AC photocurrent, “Iph, signa” is thesignal power of optical signal λ1TE or λ1TE′, and “Iph, LO” is thesignal power of light from either side S1 or S2 of laser 502.Differences in AC photocurrent generated by photodetector arrays 508-1and 508-2 may be minimized if the power of light output from side S1,when such light reaches input 507-1 of optical hybrid 506-1 compensatesdifferent AC signal power level from the two photodiode arrays 508-1 and508-2. That is, the AC powers of such mixed optical power from opticalhybrids 506-1 and 506-2 are balanced.

As further shown in FIG. 5a , coupler 504 is provided for monitoring ofLO light output from laser 502. Namely, coupler 504 has an output 504-1that provides a power split portion of the light output from side S2 toan optionally provided VOA 510, which may adjust the power of or blocksuch power split portion. If power is adjusted, the split portion of thelight output from coupler output 504-1 may be supplied to an additionalcoupler 512, which, in turn, directs such light to control circuit 514.Etalons, delay line interferometers, photodiodes and other components incontrol circuit 514 may be used to monitor LO light output from laser502 to insure that such light is locked to a particular wavelength ornarrow wavelength band, or that it has a power that is at a desiredlevel, for examples.

The second power split portion of light output from side S2 is fed toinput 507-2 of optical hybrid 506-2. Such second power split portion,however, may have a power that is less than the power of light outputfrom side S1 and fed to optical hybrid input 507-1. Referring to FIG. 5b, in order to balance of the power of the LO light supplied to inputs507-1 and 507-2, optical receiver circuit OR-1 includes a losscompensating coupler 551 having a predetermined loss so that lightpassing through such coupler at optical hybrid input 507-1 may have thesame or substantially the same power as light at input 507-2 (see FIG.5b ). Coupler 551 may be the same or similar to coupler 231 discussedabove in regard to FIG. 2c . Further, coupler 551, like coupler 231discussed above, may have a low loss or dispersive structure, such aswaveguide 551-1, which is similar to and operates in a similar fashionas waveguide 249 discussed above. Alternatively, as shown in FIG. 5g ,the waveguide 551-1 may be replaced with an absorptive termination or apower-monitoring photodiode 561-1 instead for example.

In the example shown in FIG. 5c , coupler 551 is omitted. However,reflector section R1 adjacent side S1 of laser 502 is configured to havehigher reflectivity R_(H) (more refractive index or grating “teeth”),and reflector section R2 adjacent side S2 is configured to have lowerreflectivity R_(L) (fewer grating teeth). As a result, LO light outputfrom side S2 has more power than that output from side S1. The higherpower associated with light output from side S2 may offset the loss suchlight incurs from coupler or tap coupler 504. Accordingly, LO lightoutput from coupler 504 has the same or substantially the same power atoptical hybrid input 507-2 as light output from side S1, when such S1outputted light reaches optical hybrid input 507-1.

FIG. 5d shows another example of optical receiver circuit OR-1 in whichreflector section R1 of laser 502 adjacent side S1 has a reflectivityR_(L) that is significantly less than the reflectivity R_(H) ofreflector section R2. In this example, little if any LO light is outputfrom side S2, such that all of substantially all the LO light generatedby laser 502 is output from side S1. Alternately, light from S2 may beterminated with a dispersive or absorptive element such as a photodiodethat may also monitor optical power, as shown in FIG. 5h . Referringback to FIG. 5d , light from S1 may be supplied to an optional coupler504 having first and second outputs 504-1, 504-2. LO light supplied fromoutput 504-1 may be provided to VOA 510 for monitoring purposes, asnoted above. However, LO light supplied from output 504-2 may beprovided to coupler 541. In one example, coupler 541 is a 3 dB couplerthat supplies a first portion (50%) of the received light at output541-1 and a second portion (50%) of the received light at output 541-2.As such, the light supplied from both outputs 541-1, 541-2 may be thesame or balanced. As further shown in FIG. 5d , LO light supplied fromoutput 541-1 is fed to optical hybrid input 507-1 of optical hybrid506-1 for mixing with optical signal λ1TE, and LO light supplied fromoutput 541-2 is fed to optical hybrid input 507-2 of optical hybrid506-2 for mixing with optical signal λ1TE′.

Optical receiver circuit OR-1 shown in FIG. 5e is similar to that shownin FIG. 5d , with the exception that coupler 504 is omitted and lightoutput from side S1 of laser 502 is fed directly to coupler 541 viawaveguide 593.

FIG. 5f shows an example of optical receiver circuit OR-1 having aconfiguration similar to that shown in FIG. 5c . In FIG. 5f , however,coupler output 504-1 feeds a power split portion of light output fromside S2 of laser 502 to coupler 570, which supplies a first part of thepower split portion of light to coupler 512, which, in turn, feeds suchlight to control circuit 514 for monitoring and control purposes. Thesecond part of the power split portion is supplied to coupler 572,which, along with coupler 574 and waveguide 578 constitute a delay lineinterferometer (DLI) 571, such that an output of coupler 574 may be fedto photodiode 580. Photodiode 580 may be a 1-2 bandwidth GHz photodiode(or a higher bandwidth photodiode with appropriate electronic filtering)that supplies an electrical signal characteristic of the phase noisefrom the local oscillator 502. The electrical output may then besupplied to a digital signal processor (DSP) which, based on theelectrical output, may cancel through signal processing the effects ofphase noise in the LO light that may be present in the outputs ofphotodetector arrays 508-1 and 508-2 (also coupled to the DSP). As aresult, the optical signal-to-noise ratio (OSNR) associated with thereceived optical signals, e.g., λ1TE, may be improved by 3-4 dB, forexample. Such improved OSNR may be realized with optical signalsmodulated in accordance with a 64 QAM modulation format and transmittedover conventional optical fiber. In one example, DLI 571 may be providedas part of PIC 406 on substrate 408 (see FIG. 4).

As further shown in FIG. 5f , a low loss or dispersive structure may beprovided at an output of coupler 574, which may be an MMI coupler. Suchlow loss or dispersive structure may constitute waveguide 576, which maybe the same or similar to waveguides 249 and 551-1 discussed above.

Various power balancing techniques are discussed above to provide thesame or substantially the same optical power to devices in both atransmitter and a coherent receiver. Consistent with a further aspect ofthe present disclosure, the light output from a laser may becharacterized or tested using a “shutter” VOA, as discussed in greaterdetail below with respect to FIGS. 6-11. Generally, the examples shownin FIGS. 6-11 enable a characterization of a laser output from the LOlaser using a single path on a PIC.

FIG. 6 shows an example of a PIC 408 provided on substrate 406 (see FIG.4). As noted above, PIC 408 may include optical demultiplexer circuitry.Such circuitry is shown in FIG. 6 including a first splitter or opticaldemultiplexer 410-1, including a first arrayed waveguide grating, forexample, and a second splitter or optical demultiplexer 410-2, includinga second arrayed waveguide grating, for example. As further noted above,optical signals λ1TE . . . λnTE are supplied to demultiplexer circuitry410 from a first output of PBS 402 and optical signals λ1TE′ . . . λnTE′are output from polarization rotator 404, for example. Here, opticalsignals λ1TE . . . λnTE are input to splitter or demultiplexer 410-1 andoptical signals λ1TE′ . . . λnTE′ are input to splitter or demultiplexer410-2. Demultiplexer 410-1 has a plurality of outputs 411, one of whichsupplies optical signal λ1TE to optical hybrid 506-1, and each of theremaining optical signals optical signals λ2TE . . . λnTE is provided toa respective first optical hybrid input of each of optical receivers OR2to ORn. Similarly, demultiplexer 410-2 has a plurality of outputs 413,one of which supplies optical signal λ1TE′ to optical hybrid 506-2, andeach of the remaining optical signals optical signals λ2TE . . . λnTE isprovided to a respective first optical hybrid input of each of opticalreceivers OR2 to ORn.

As further shown in FIG. 6, OR-1 may include LO laser 502, as in FIGS.5a to 5f described above. Laser 502 supplies first LO light 672 to input507-1 and second LO light 662 to coupler 624. Coupler 624 may be a 10/90tap that provides a first portion 663 of LO light of LO light to SVOA626 and a second portion 664 of LO light to input 507-2 of opticalhybrid 506-2. In a manner similar to that discussed above, LO lightsupplied from laser 502 may be mixed in optical hybrids 506-1 and 506-2and the resulting mixing products may be provided to respectivephotodetector arrays 508-1 and 508-2.

PIC 408 shown in FIG. 6 may be operated in first and second modes. Inthe first or test mode, laser 622 is powered on and SVOA 626 is biasedto be “open”, such that SVOA 626 is substantially or entirelytransmissive or even supply gain. As a result, the first portion 663 ofLO light 662 is supplied via SVOA 626 to analyzer circuit 628 formonitoring one or more parameters of the LO light, such as wavelength,laser linewidth, and power. During a second or operational mode, SVOA626 is biased to be substantially opaque to the LO light, such that suchlight is blocked from reaching analyzer circuit 628. In the operationalmode, as noted above, optical signals λ1TE and λ1TE′ are mixed with LOlight 672 and 664, respectively, in corresponding optical hybridcircuits 506-1 and 506-2. To block the light may mean blocking 90%, 99%,99.9%, 99.99%, or any suitable number.

FIG. 7 shows an example of optical receiver circuit OR-1 similar to thatshown in FIG. 6. In FIG. 7, however, demultiplexers 410-1 and 410-2 areomitted, such that OR-1 and PIC 408 receive one optical signal, such asλ1TE and λ1TE′. The operation and structure of optical receiver circuitOR-1 shown in FIG. 7 is otherwise the same as or similar to theoperation and structure of OR-1 discussed above in connection with FIG.6.

FIG. 8 illustrates receiver PIC 800 including receivers 801 and 802 andrespective local oscillator lasers 830 and 840 consistent with anadditional aspect of the present disclosure. PIC 800 also includesshutter VOAs 810 and 812. PIC 800 may operate in one of either a testmode or a normal operational mode as described in above with referenceto FIGS. 6 and 7.

Receiver 801 includes a LO laser 830, in which one output of the LOlaser 830 is coupled to an optical tap 809 and another output of the LOlaser 830 is coupled to an optical hybrid 824. A portion of the tappedlight from the optical tap 809 is provided to an optical hybrid 822, andanother portion of the tapped light from the optical tap 809 is providedto SVOA 810. A data signal (e.g., λ1TE) may be coupled into the PIC 800through the 3 dB coupler 803, where a portion the data signal (e.g.,50%) is provided to the optical hybrid 822 and another portion (e.g.,50%) the data signal is provided to the optical hybrid 826. Tap 811sends light through coupler 804 for test analysis. In this way, ACphotocurrent is balanced by tapping the local oscillator path on oneside and the signal path on the other side rather than tapping both onthe one side and not at all on the other side. Control electrodes 891may be provided to adjust the current supplied to heaters that controlthe temperature of the reflectors (shown as mirror1 and mirror2) of LOlaser 830, as well as the current supplied to the gain and phasesections of LO laser 830. LO laser 830 is shown, in this examples, ashaving a bent or curved phase adjusting section φ. With this geometry,LO laser 830 and PIC 800 may have a compact layout and chip size.

PIC 800 also includes receiver 802, which has a similar construction asreceiver 801. For example, receiver 802 includes an LO laser 840 havingfirst and second outputs adjacent mirror or reflectors mirror1 andmirror2, respectively. The output from mirror1 is supplied to opticaltap 814 and the second output of the LO laser 840 from mirror2 iscoupled to optical hybrid 828. Control electrodes 892, similar toelectrodes 891, may also be provided to adjust the gain and thetemperature of the reflector sections (mirror1 and mirror2) of LO laser840. A portion of the tapped light from the optical tap 814 may beprovided to optical hybrid 826, and another portion of the tapped lightfrom the optical tap 814 is provided to SVOA 612. A second data signal(e.g., λ1TE′) may be coupled into the PIC 800 through the 3 dB coupler804, where a portion the data signal λ1TE′ (e.g., 50%) is provided tothe optical hybrid 824 through the tap 813 and another portion (e.g.,50%) the data signal λ1TE′ is provided to the optical hybrid 828 throughthe optical tap 813.

Alternatively, light from one end of laser 840 may be split by a 3 dBcoupler, for example, having first and second output ports. Light fromthe first output port may be supplied to tap 814 and light from thesecond output port may be provided to tap 828.

In this example, in the test mode, the SVOA 810 may be biased to be“open”. Light output from mirror1 of laser 830 may be split by anoptical tap 809, and the split portion 831 may pass through the SVOA810, another tap 811, and a 3 dB coupler 804 where such light may bedetected on-wafer, on-chip or off-chip in order to determine that lightoutput from the LO laser 830 has a desired wavelength and power. In someimplementations, other performance metrics of the LO laser 830 may alsobe detected and analyzed.

In a similar fashion, SVOA 812, which may have a structure similar to orthe same as SVOA 810 may be biased in a test mode to transmit light fromthe LO laser 840 via tap 814. Light output from the SVOA 812 may passthrough another tap 813 and the 3 dB coupler 804 for monitoring, asdescribed above. In this example, the optical outputs from both the LOlasers 830 and 840 may be monitored by a single port, e.g., the outputport of the 3 dB coupler 804.

In a normal operational mode, SVOA 810 and SVOA 812 are biased to beblocking or substantially opaque. Incoming optical signals λ1TE andλ1TE′, for example, are provided to the 3 dB couplers 803 and 804,respectively. As a result, power split portions of λ1TE are supplied to90 degree optical hybrids 822,824 in receiver 801, and power splitportions of λ1TE′ are supplied to 90 degree optical hybrids 826, 828 ofoptical receiver 802. λ1TE portions 641, 642 are thus mixed with LOlight 833 and 834. In addition, λ1TE′ portions 643 and 644 are mixedwith LO light 635 and 636. The resulting mixing products are fed tophotodiodes groups 605, 606, 607, and 608, where the mixing products areconverted to corresponding electrical signals, which are then processedfurther (e.g., using coherent detection processing).

In some implementations, the receivers 801 and 802 include fiberalignment devices, such as alignment lasers 921 and 924, respectively,that supply light to corresponding couplers, such as multimodeinterference (MMI) couplers 932 and 923. Light from alignment lasers 921and 924 may then be passed through taps 811 and 813, respectively andthrough 3 dB coupler 804. Detectors spaced from coupler 804 may then beprovided to sense such light to insure proper alignment between the PIC800 and other devices.

FIG. 9 illustrates an example receiver PIC 900 for on-chip/wafertesting. The PIC 900 is similar to the PIC 800 discussed above withreference to FIG. 8. FIG. 10, however, shows an enlarged view of a waferincluding scribe lines 1031 and 1032 extending parallel to one anotherand horizontally, as well as scribe lines 1035 and 1036 that extendparallel to one another and vertically. The scribe lines define dies ordie regions 1037, 1038, and 1039. Die region 1038 is shown havingreceivers 801 and 802 having the same or similar structure and operationas receivers 801 and 802 discussed above in connection with FIG. 9. Itis understood that die regions 1037 and 1039 may have receivers similarto or the same as 801 and 802 in FIG. 10, and that other receivers (notshown) may be provided in these die regions.

As further shown in FIG. 9, waveguide 1040 may be provided in die region1037, such that waveguide 1040 loops back to optically connect 3 dBcoupler 803 to 3 dB coupler 804. In order to test waveguide continuityand operation of the photodiodes, the local oscillator lasers 830 and840 of FIG. 10 may supply light that is output from 3 dB coupler 804 ina manner similar to that described above. In addition to or instead ofsuch LO light, alignment lasers 921 and 923 may be turned on to supplylight that may also be output from 3 dB coupler 804. Light output from 3dB coupler 804 may be fed to the waveguide 1040 which supplies the lightto 3 dB coupler 803, which, in turn, directs the light to the 90 degreeoptical hybrids 822 and 866 in receivers 801 and 802, respectively. Suchlight may then pass through the optical hybrids 822 and 826 and to thephotodiodes 805 and 807 in each receiver, where the light may bedetected for monitoring, alignment, and/or diagnostic purposes.Waveguide 1042 may also be provided to facilitate such monitoring,alignment, and diagnostics of receivers in die region 1039.

To package the PIC 1000, the wafer may be scribed or cleaved alongsingulation lines 1031, 1032, 1034, and 1035, and 3 dB couplers 803 and804 may be optically coupled to receive incoming TE and TE′ opticalsignals.

Other embodiments will be apparent to those skilled in the art fromconsideration of the specification. It is intended that thespecification and examples be considered as exemplary only.

For example, in the above examples, optical signals generated bytransmit blocks 12-1 to 12-n are combined onto one optical communicationpath or fiber 16 in FIG. 1. Such optical signals are then demultiplexedor separated from one another at receive node 18. Consistent with anaspect of the present disclosure, however, polarization multiplexedoptical signals, each at a corresponding one of a plurality ofwavelengths, may be transmitted on a respective one of a plurality offibers in a so-called N×N configuration. For example, system 1000 shownin FIG. 10 may include optical sources OS-1 to OS-n having the same orsimilar structure and operating in the same or in a similar fashion asthe optical source discussed above. As further noted above, opticalsources OS-1 to OSn may output pairs of optical signals at eachwavelength λ1 to λn. λ1TE and λ1TE′, as well as λnTE and λnTE′, areexamples of such pairs. The polarization of each of optical signalsλ1TE′ to λnTE′ may be supplied to a corresponding one of firstpolarization rotators R1-1 to R1-n, each of which polarization rotatesthe received optical signal to have a TM polarization. As a result, eachof optical signals λ1TM to λnTM is output from a corresponding one ofrotators R1-1 to R1-n. Optical signals pairs λ1TE, λ1TM . . . λnTE, λnTMare next fed to a respective one of polarization beam combiners PBC-1 toPBC-n, and the polarization multiplexed optical signal (having both TEand TM components) output from each polarization beam combiner, i.e., acorresponding one of optical signals λ1(TE+TM) . . . λn(TE+TM), issupplied to a respective one of optical fibers F1 to Fn, for example.

At a receive end of each optical fiber F1 to Fn, each of optical signalsλ1(TE+TM) . . . λn(TE+TM) is supplied to an input IN corresponding oneof polarization beam splitters PBS-1 to PBS-n. Each of polarization beamsplitters PBS-1 to PBS-n supplies the TM component, e.g., λ1TM, of eachsuch optical signal at output OUT1 and the TE component of each suchoptical signal, e.g., λ1TE, at output OUT2 of each PBS. Each of TMcomponents λ1TM to λnTM is provided to a respective one of secondrotators R2-1 to R2-n, which rotate the polarization of each such TMcomponent to have a TE polarization. Each of the resulting opticalsignals λ1TE′ to λnTE′, along with a respective one of optical signalsλ1TE to λnTE, are supplied to corresponding one of optical receiversOR-1 to OR-n, which process such optical signal in a manner similar toor the same as that described above.

While this document may describe many specifics, these should not beconstrued as limitations on the scope of an invention that is claimed orof what may be claimed, but rather as descriptions of features specificto particular embodiments. Certain features that are described in thisdocument in the context of separate embodiments can also be implementedin combination in a single embodiment. Conversely, various features thatare described in the context of a single embodiment can also beimplemented in multiple embodiments separately or in any suitablesub-combination. Moreover, although features may be described above asacting in certain combinations and even initially claimed as such, oneor more features from a claimed combination can in some cases be excisedfrom the combination, and the claimed combination may be directed to asub-combination or a variation of a sub-combination. Similarly, whileoperations are depicted in the drawings in a particular order, thisshould not be understood as requiring that such operations be performedin the particular order shown or in sequential order, or that allillustrated operations be performed, to achieve desirable results.

Only a few examples and implementations are disclosed. Variations,modifications, and enhancements to the described examples andimplementations and other implementations can be made based on what isdisclosed.

What is claimed is:
 1. An optical source, comprising: a substrate; a laser provided on the substrate, the laser having first and second sides and outputting first light from the first side and second light from the second side, the first light output from the first side of the laser has a first power and the second light output from the second side has a second power; and a first modulator that receives the first light and a second modulator that receives the second light, such that the power of the first light at an input of the first modulator is substantially equal to the power of the second light at an input of the second modulator.
 2. An optical source in accordance with claim 1, wherein the first modulator includes a first Mach-Zehnder modulator and the second modulator includes a second Mach-Zehnder modulator.
 3. An optical source in accordance with claim 1, further including an optical coupler optically communicating with the first side of the laser and receiving the first light.
 4. An optical source in accordance with claim 3, wherein the optical coupler has an associated optical loss, the second power of the second light output from the second side being greater than the first power of the first light output from the first side, the second power offsetting the optical loss.
 5. An optical source in accordance with claim 3, wherein the coupler has first and second outputs, the first modulator optically communicating with the first output, the optical source further including a device coupled to the second output.
 6. An optical source in accordance with claim 5, wherein the device includes a waveguide that disperses a portion of the first light supplied from the second output of the coupler.
 7. An optical source in accordance with claim 3, wherein the coupler is a multimode interference (MMI) coupler.
 8. An optical source in accordance with claim 1, wherein the laser includes a first reflector adjacent the first side and a second reflector adjacent the second sides, the first reflector having a first reflectivity and the second reflector having a second reflectivity, such that the first power of the first light output from the first side is different than the second power of the second light output from the second side.
 9. An optical source in accordance with claim 8, wherein the first reflector has a first plurality of grating teeth and the second reflector has a second plurality of grating teeth, a number of the first plurality of grating teeth being different than a number of the second plurality of grating teeth.
 10. Optical receiver, comprising: a substrate; a local oscillator (LO) laser provided on the substrate, the LO laser having first and second sides and outputting first light from the first side and second light from the second side, the first light output from the first side of the laser has a first power and the second light output from the second side has a second power; a first plurality of photodiodes receiving first mixing products including portions of the first light and portions of a first optical signal [TE signal, for example]; a second plurality of photodiodes receiving second mixing products including portions of the second light and portions of a second optical signal [the TE′ signal, for example]; a first optical path extending from the first side of the LO laser to the first plurality of photodiodes and a second optical path extending from the second side of the LO laser to the second plurality of photodiodes, the first path having an associated first optical loss and the second path having an associated second optical loss, wherein the first light has a first power and the second light has a second power at the first and second sides, respectively, such that portions of the first light received by the first plurality of photodiodes have substantially the same power as portions of the second light received by the second plurality of photodiodes.
 11. An optical receiver in accordance with claim 10, further including an optical coupler optically communicating with the first side of the LO laser and receiving the first light.
 12. An optical receiver in accordance with claim 11, wherein the optical coupler has an associated optical loss, the second power of the second light output from the second side being greater than the first power of the first light output from the first side, the second power offsetting the optical loss.
 13. An optical receiver in accordance with claim 11, wherein the coupler has first and second outputs, the first plurality of photodiodes optically communicating with the first output, the optical receiver further including a device coupled to the second output.
 14. An optical receiver in accordance with claim 13, wherein the device includes a waveguide that disperses a portion of the first light supplied from the second output of the coupler.
 15. An optical receiver in accordance with claim 11, wherein the coupler is a multimode interference (MMI) coupler.
 16. An optical receiver in accordance with claim 10, wherein the LO laser includes a first reflector adjacent the first side and a second reflector adjacent the second sides, the first reflector having a first reflectivity and the second reflector having a second reflectivity.
 17. An optical receiver in accordance with claim 16, wherein the first reflector has a first plurality of grating teeth and the second reflector has a second plurality of grating teeth, a number of the first plurality of grating teeth being different than a number of the second plurality of grating teeth.
 18. A receiver in accordance with claim 10, further including a coupler coupled along the second path, the coupler having a first output and a second output, the first output optically communicating with a control circuit that monitors the second light, and the second output optically communicates with the second plurality of photodiodes.
 19. A receiver in accordance with claim 18, wherein the coupler [the tap] is a first coupler further including: a variable optical attenuator optically communicating with the first output of the first coupler; and a second coupler having an input that optically communicates with variable optical attenuator and an output that optically communicates with the control circuit.
 20. A receiver in accordance with claim 18, further including: a delay line interferometer provided on the substrate, the delay line interferometer having an input that optically communicates with the first output of the coupler and an output; and a photodetector that optically communicates with the output of the delay line interferometer, wherein a digital signal processor offsets effects of phase noise in the first and second lights based on an output of the photodetector.
 21. An optical receiver in accordance with claim 10, further including a first optical hybrid circuit provided along the first optical path and a second optical hybrid circuit provided along the second optical path.
 22. An optical receiver, comprising: a local oscillator (LO) laser having first and second sides and outputting first light from the first side and second light from the second side; a coupler that receives the first light, the coupler having first and second outputs, the first output supplying a first portion of the first light and the second output supplying a second portion of the first light; a first optical hybrid that receives the first portion of the first light; a second optical hybrid that receives the second portion of the first light; and a control circuit optically communicating with the second side of the LO laser, the control circuit receiving at least a portion of the second light and monitoring the second light.
 23. An optical receiver in accordance with claim 22, wherein the LO laser includes a first reflector adjacent the first side and a second reflector adjacent the second side, the first reflector having a reflectivity that is less than a reflectivity of the second reflector.
 24. An optical receiver, comprising: a local oscillator (LO) laser that output light; a first coupler [monitoring tap] that receives the light, the coupler having first and second outputs, the first output supplying a first portion of the light and the second output supplying a second portion of the light; a second coupler [the 3 dB coupler in FIG. 5d ] having an input that receives the first portion of the light, the second coupler having a first output that supplies a third portion of the light and a second output that supplies a fourth portion of the light; a first optical hybrid circuit that receives the third portion of the light; a second optical hybrid circuit that receives the fourth portion of the light; and a control circuit optically communicating with the second output of the first coupler, the control circuit receiving at least a portion of the second portion of the light, the control circuit monitoring the second portion of the light.
 25. An optical system, comprising: a plurality of optical transmitters, each including a corresponding one of a first plurality of lasers, each of the first plurality of lasers having a first side and a second side, such that first light is supplied from the first side and second light is supplies from the second side, a power of the first light being substantially the same as a power of the second light when output from each the plurality of optical transmitters; and an optical combiner that optically communicates with each of the plurality of transmitters and supplies optical signals including the first and second lights from each of the plurality of transmitters onto an optical communication path.
 26. An optical system, comprising: a plurality of optical transmitters, each including a corresponding one of a first plurality of lasers, each of the first plurality of lasers having a first side and a second side, such that first light is supplied from the first side and second light is supplies from the second side, a power of the first light being substantially the same as a power of the second light when output from each the plurality of optical transmitters; a plurality of optical fibers, each of which optically communicating with a corresponding one of the plurality of optical transmitters, such that each of the optical fibers carries an optical signal output from a respective one of the plurality of optical transmitters, each said optical signal including the first light and the second light from a corresponding one of the plurality of optical transmitters. 